The corticobulbar tract conducts impulses from the brain to the cranial nerves.[1] These nerves control the muscles of the face and neck and are involved in facial expression, mastication, swallowing, and other functions.

The corticospinal tract conducts impulses from the brain to the spinal cord. It is made up of a lateral and anterior tract. The corticospinal tract is involved in voluntary movement. The majority of fibres of the corticospinal tract cross over in the medulla, resulting in muscles being controlled by the opposite side of the brain. The corticospinal tract also contains Betz cells (the largest pyramidal cells), which are not found in any other region of the body.

The pyramidal tracts are named because they pass through the pyramids of the medulla. The corticospinal fibers when descending from the internal capsule to the brain stem, converge to a point from multiple directions giving the impression of inverted pyramid.

The myelination of the pyramidal fibres is incomplete at birth and gradually progresses in caudo-cranial direction and thereby progressively gaining functionality. Most of the myelination is complete by two years of age and thereafter it progresses very slowly in cranio-caudal direction up to twelve years of age.

The nerve axons traveling down the tract are referred to as upper motor neurons. These axons travel down the tracts in the white matter of the spinal cord until they reach the vertebral level of the muscle that they will innervate.[5] At this point, the axons synapse with lower motor neurons. The majority of axons do not directly synapse with lower motor neurons, but instead synapse with an interneuron that then synapses with a lower motor neuron. This generally occurs in the anterior horn of the spinal cord.[2] Nerve axons of the lateral corticospinal tract that did not cross over in the medulla do so at the level of the spinal cord they terminate in.[6]

These tracts contain more than 1 million axons and the majority of the axons are myelinated. The corticospinal tracts myelinate largely during the first and second years after birth. The majority of nerve axons are small (<4μm) in diameter. About 3% of nerve axons have a much larger diameter (16μm) and arise from Betz cells, mostly in the leg area of the primary motor cortex. These cells are notable because of their rapid conduction rate, over 70m/sec, the fastest conduction of any signals from the brain to the spinal cord.[2]

Horizontal section through the lower part of the pons, showing the fibers of the corticospinal tract (#19) passing through the pontine nuclei

The nerves within the corticospinal tract are involved in movement of muscles of the body. Because of the crossing-over of fibres, muscles are supplied by the side of the brain opposite to that of the muscle.[2] The nerves within the corticobulbar tract are involved in movement in muscles of the head. They are involved in swallowing, phonation, and movements of the tongue.[6] By virtue of involvement with the facial nerve, the corticobulbar tract is also responsible for transmitting facial expression.[5] With the exception of lower muscles of facial expression, all functions of the corticobulbar tract involve inputs from both sides of the brain.[5]

If the corticobulbar tract is damaged on only one side, then only the lower face will be affected, however if there is involvement of both the left and right tracts, then the result is pseudobulbar palsy. This causes problems with swallowing, speaking, and emotional lability.[4]